Unbound is a validating, recursive, and caching DNS resolver.
Currently it’s only used only as DNS server for anti-spam DNSBL implementation.

When a new mail arrives, the anti-spam filter checks if sender IP address is listed
in well-known blacklists.
Some blacklists doesn’t allow multiple query from public DNS servers,
thus the system needs a DNS server which can directly query the blacklist DNS.

If the mail filter is installed, dnsmasq configuration is changed, and specific
domain query are redirect trough unbond.
See: /etc/e-smith/templates/etc/dnsmasq.conf/26unbound_rbl.